After researching magazine fixes here in this forum I found the following issue with my magazine:

The areas pointed out by the top two arrows are the contours of the feed lips which appear to properly confirm to the cartridge contours.

The areas pointed out by the bottom two arrows are the "throat" of the feed area which I found to be too tight so I opened them up a bit with needle nose pliers and polished all the edges with a Dremel.

Dummy rounds now glide out of the magazine effortlessly with thumb pressure, which they didn't before.

I'll be making another trip to the range today and will report back here...

Tore the Tommy down for cleaning and found a few rough spots in the receiver.

Polished them out as best as I could.

The photo shows the area between the bolt cavity and the magazine feed area (or whatever you call it). The bolt "finger" hits this "step" and causes a slight "bump" when closing. I was able to smooth it out and make a small "ramp" instead of a square shoulder. Hopefully this will aid in smoothing out bolt sliding.

Also was able to polish the feed ramp leading to the chamber. Not perfect, but much smoother now.

you are doing what most of us have already done. I bought another tommy a few months back. had the extractor popping out. replaced with a new one ,same issue. then I staked it. 400 rounds zero issues. its minor stuff that just needs to be tuned. what ammo are you using? I use the Winchester brown box.

I feel your pain, OP. I had my Thompson out to the range today. It ran great as expected. Until I got to a new mag I hadn't used before. With that mag, it wouldn't fire more than 3 rounds in a row without jamming. All of mine, the round jammed "sideways", where it favored one side rather than straight up and down.